Quantum effects in a double-walled carbon nanotube capacitor

We have theoretically investigated the capacitance of a double-walled carbon nanotube (CNT) (4,0)@(24,0) as a nanoscale coaxial-cylindrical capacitor. To calculate the responses of the capacitor to the bias application, we have used a first-principles method based on the density-functional theory with a local-density approximation. We show that the capacitance exhibits two principal quantum effects: First, the capacitance shows a large bias dependence, reflecting the density of states of the CNT electrodes. Second, the capacitance is enhanced according to a quantum-mechanical spill of the stored electron density from the tube walls of the CNTs. We argue that these two quantum effects are fundamental factors in nanoscale capacitors.